Gate dielectric. As Figure 1 shows, the gate dielectric traditionally has been made of silicon dioxide, a poor conductor of electricity that prevents the leakage of electrons from the gate to the substrate. Over time, Intel has reduced the silicon-dioxide gate dielectric's thickness to 1.2 nm—five atomic layers—for chips with 65-nm feature sizes.
However, electrons increasingly leak through such a thin layer. This wastes energy. And, when the electron flow hits the dielectric, the resistance generates heat.
To cope with these problems, AMD, IBM, and Intel have experimented with making gate dielectrics using high-k materials, including hafnium. ("k" represents the dielectric constant, which describes a material's ability to transmit charge when a voltage is applied.)
Jim Hutchby, director of device sciences at Semiconductor Research Corp., an industry research-management consortium, said AMD, IBM, and Intel probably are using nitride of hafnium silicate consisting of hafnium, silicon, oxygen, and nitrogen. However, AMD, IBM, and Intel are not disclosing the exact mix of materials used in their new designs.
As transistors have become smaller horizontally, they have also had to cope with narrower transistor channels. To provide the current necessary for the system to accurately read high and low levels, the smaller systems must get more electrostatic charge through the dielectric layer to the channel.
High-k materials can transmit more charge than silicon dioxide. And building the dielectric thicker vertically would provide more resistance, which reduces leakage without making transistors bigger horizontally.
Scientists have researched the use of high-k materials in transistors for about two decades, noted UC Berkeley's Subramanian. However, the material decreased performance by reducing the ability of electrons in the channel to move around and thereby increase current flow through the channel.
Researchers minimized this problem by adding an intervening layer of silicon oxide. The material reduced the effect of thermal currents between the transistor channel and the dielectric, which inhibit electron mobility
Gates. Transistor gates currently are made of layers of polysilicon—doped noncrystallized silicon—deposited onto the substrate.
High-k dielectrics can't be used with conventional polysilicon gate electrodes because the two materials don't match well electromagnetically. This keeps the current flow across the transistor channel from switching as quickly as necessary.
To cope with this, AMD, IBM, and Intel have developed metal gate electrodes, which match better electromagnetically with the high-k materials.